1. Field of the Invention
The present invention relates to a wire-wound coil, and more particularly, to a wire-wound coil for use in, for example, an inductor, a common-mode choke coil, a normal-mode choke coil, a transformer, or other suitable device.
2. Description of the Related Art
In general, the insertion loss versus frequency characteristic of a common-mode choke coil is significantly influenced by an inductance component due to the common-mode inductance L in the region of frequencies lower than the self-resonant frequency, and is significantly influenced by a capacitance component due to the stray capacitance C produced in the common-mode choke coil in the region of frequencies higher than the self-resonant frequency. The self-resonant frequency measured when the impedance is about 50 xcexa9 is represented by the following Expression f0, the insertion loss versus frequency characteristic in the region of frequencies lower than the self-resonant frequency is represented by the following Approximate Expression 1, and the insertion loss versus frequency characteristic in the region of frequencies higher than the self-resonant frequency is represented by the following Approximate Expression 2:
f0:fr=1/[2xcfx80(LC)1/2]
Approximate Equation 1:
insertion loss=10 log [1+(xcfx89L/100)2]
Approximate Equation 2:
insertion loss=10 log [1+1/(100xcfx89C)2]
In order to improve the noise-eliminating performance of the common-mode choke coil in the high-frequency region, the stray capacitance C must be decreased. The stray capacitance C is principally caused by the influences of a winding structure of windings, bobbins, and a magnetic core. In order to reduce the influence of the bobbins, it is necessary to change the material of the bobbins to a material having a lower dielectric constant, or to reduce the thickness of the bobbins. However, when the common-mode choke coil is used for an AC supply line, flame retardancy, relative thermal index, an insulation distance according to the safety standards must be ensured. Since existing common-mode choke coils generally adopt thick bobbins having a thickness of 0.5 mm to 1.0 mm and are made of a material having a dielectric constant ∈ of 2 to 4, it is difficult to reduce the influence of the bobbins on the stray capacitance C by changing the material and thickness of the bobbins.
Accordingly, in order to reduce the stray capacitance C produced in the common-mode choke coil, it is important to reduce the influence of the winding structure of the windings, and the influence of the magnetic core. The ratio of the influences varies depending on the winding structure of the windings. For example, so-called sectional winding for winding windings in sections is known as a winding structure that produces little stray capacitance.
FIG. 21 shows the configuration of a known common-mode choke coil 1 in which windings 7 and 17 are wound in sections. The common-mode choke coil 1 includes a magnetic core constituted by two U-shaped core members 20 and 21, and two bobbins 2 and 12. The bobbins 2 and 12 include cylindrical body portions 3 and 13, and flange portions 4, 5, and 6, and 14, 15, and 16 provided in the cylindrical body portions 3 and 13, respectively.
The winding 7 is formed by electrically connecting a first winding portion 7a and a second winding portion 7b in series. The first winding portion 7a is wound between the flange portions 4 and 6 of the bobbin 2, and the second winding portion 7b is wound between the flange portions 5 and 6. Similarly, the winding 17 is formed by electrically connecting a first winding portion 17a and a second winding portion 17b in series. The first winding portion 17a is wound between the flange portions 14 and 16 of the bobbin 12, and the second winding portion 17b is wound between the flange portions 15 and 16.
The bobbins 2 and 12 are arranged so that the cylindrical body portions 3 and 13 thereof are parallel to each other. Leg portions 20b and 21b of the core members 20 and 21 extend in holes 3a and 13a of the cylindrical body portions 3 and 13, respectively. The core members 20 and 21 define one closed magnetic circuit with the leading end surfaces of the leg portions 20b and 21 abutting against each other inside the holes 3a and 13a. 
In the common-mode choke coil 1 having the above-described configuration, since the stray capacitance is substantially proportional to the winding width, when the windings 7 and 17 are divided into the two winding portions 7a and 7b and the two winding portions 17a and 17b, respectively, the stray capacitance of one winding portion is half the stray capacitance of the undivided winding.
Since the winding portions 7a and 7b, or the winding portions 17 and 17b are connected in series, the stray capacitance of each of the windings 7 and 17 in the two-section winding common-mode choke coil 1 is one fourth of the stray capacitance of the undivided winding (for example, approximately 4.0 pF).
Another winding structure is a so-called single-layer winding structure in which a winding is wound only in one layer. In this winding structure, the turns are adjacent only in the lateral direction, and a number of stray capacitances produced in the adjacent turns corresponding to the number of turns are connected in series, which can minimize the stray capacitance. For example, the stray capacitance (4.0 pF) in the above-described sectional winding can be reduced to approximately one-sixth or less by the single-layer winding. However, the inductance obtained in this case is low.
A so-called single-layer multiple winding structure is also known in which a plurality of single-layer windings are stacked in parallel. In order to overcome the problem of low inductance in the single-layer winding structure, in this winding structure, the diameter of the wire is decreased, and the number of turns in each layer of the winding is increased, thereby increasing the inductance. Since the direct resistance of the windings is thereby increased, a plurality of stacked layers of windings are connected in parallel. That is, the single-layer multiple winding structure has characteristics similar to those of the single-layer winding structure, and also achieves a relatively high inductance. However, the stray capacitance is higher than in the single-layer winding structure.
Table 1 shows the general differences of the stray capacitance, the direct resistance of the winding, and the inductance among the above-described winding structures when the wire diameter is not changed.
In general, the areas in which the windings 7 and 17 of the common-mode choke coil 1 can be wound are limited by, for example, the planar area of the space defined by the inner peripheries of the core members 20 and 21 that define the closed magnetic circuit, the thickness of the bobbins 2 and 12, and the insulation distance. The known common-mode choke coil 1 is designed so that there is no wasted space, in order to achieve the maximum possible inductance in the limited winding areas. Therefore, only the minimum gaps required for assembly operation and safety standards are formed between the core members 20 and 21 and the bobbins 2 and 12, or between the core members 20 and 21 and the windings 7 and 17. Consequently, the stray capacitance produced by the core members 20 and 21 is relatively high. In the common-mode choke coil 1 in which the windings 7 and 17 are wound in a manner that produces less stray capacitance than the multiple winding common-mode choke coil which does not have the center flange portions 6 and 16 for dividing the windings 7 and 17, the influence of the stray capacitance is not negligible. In particular, in the single-layer winding structure and the single-layer multiple winding structure that produce little stray capacitance, the influence of the core members 20 and 21 on the stray capacitance is quite significant.
In order to overcome the problems described above, preferred embodiments of the present invention provide a wire-wound coil having a structure that minimizes the influence of a magnetic core on the stray capacitance.
According to a preferred embodiment of the present invention, a wire-wound coil includes one or more bobbins each having a substantially cylindrical body portion and a flange portion disposed on the substantially cylindrical body portion, one of a single-layer winding and a single-layer multiple winding wound on the substantially cylindrical body portion of each of the bobbins, and a magnetic core having an arm portion and a leg portion extending in a hole formed in the substantially cylindrical body portion of each of the bobbins so as to define a closed magnetic circuit, wherein a first gap is formed between the inner peripheral surface of the hole of the substantially cylindrical body portion of each of the bobbins and the outer peripheral surface of the leg portion of the magnetic core, and a second gap is formed between the flange portion of each of the bobbins and the arm portion of the magnetic core facing the flange portion.
The first gap is formed, for example, by a rail-shaped rib disposed on at least one of the inner peripheral surface of the hole of the substantially cylindrical body portion of each of the bobbins and the outer peripheral surface of the leg portion of the magnetic core. The second gap is formed, for example, by a convex spacer disposed on at least one of the flange portion and the leg portion of the magnetic core facing the flange portion. Preferably, the first gap is about 0.3 mm to about 1.5 mm, and the second gap is about 0.7 mm to about 4.0 mm.
With the unique configuration as described above, the gaps of predetermined lengths are ensured between the magnetic core and the winding, and the distance therebetween is increased. This reduces the influence of the magnetic core on the stray capacitance. As a result, it is possible to achieve a wire-wound coil having superior electrical characteristics in the high-frequency region.
By placing an insulating resin member including magnetic powder or a ferrite member covered with insulating resin between two adjoining bobbins, the effective magnetic permeability of the normal-mode magnetic circuit is increased, and the normal-mode inductance is increased. Moreover, since magnetic flux is concentrated by the insulating member including magnetic powder or the ferrite member covered with insulating resin, magnetic flux does not leak to the outside.
Further elements, characteristics, features, and advantages of the present invention will become apparent from the following description of preferred embodiments with reference to the attached drawings.